Products or components that should not have a leak when used have been conventionally checked in their production lines for their acceptability.
FIG. 8 is a block diagram showing a general structure of a leak inspector used for such a check. A flow pipe 10 connected to the outlet side of a pneumatic source 11 is extended through a pressure control valve 12 and a three-way solenoid valve 14 and branches into branch pipes 15A and 15B at the outlet side of the three-way solenoid valve 14. Between the outlet side of the pressure control valve 12 and the inlet side of the three-way solenoid valve 14, a pressure gauge 13 for monitoring a specified inspection pressure is connected.
The branch pipe 15A is connected to one end of a guide pipe 18 through a solenoid valve 16, and the other end of the guide pipe 18 is connected a connection jig 24 that can be connected to a device 20 under inspection to be checked for a leak. Devices 20 under inspection are sequentially connected to the connection jig 24 to allow leak inspection therefor.
The branch pipe 15B is connected to one end of a guide pipe 19 through a solenoid valve 17, and the other end of the guide pipe 19 is connected to a reference tank 21. The guide pipes 18 and 19 are respectively connected to guide pipes 18A and 19A, as branches, and a pressure-difference detector 22 is connected between the guide pipes 18A and 19A.
The output signal of the pressure-difference detector 22 is sent to a comparator 32 through an automatic-zero-reset amplifier 31, and can be compared in the comparator 32 with a reference value RV given by a reference-value setter 33.
The device 20 is connected to the end of the guide pipe 18; the reference tank 21, having no leak, is connected to the guide pipe 19; the three-way solenoid valve 14 is closed between a and b; and the pressure control valve 12 is adjusted while the pressure gauge 13 is monitored, to provide a predetermined air pressure from the pneumatic source 11. Then, the solenoid valves 16 and 17 are opened; the three-way solenoid valve 14 is opened between a and b; and the specified constant air pressure is applied to the device 20 and the reference tank 21 through the branch pipes 15A and 15B and the guide pipes 18 and 19, respectively.
When the pressure in the device 20 and the reference tank 21 becomes stable after a predetermined period of time elapses, the solenoid valves 16 and 17 are closed. Then, after a predetermined stabilizing period of time (equilibrium time) further elapses, the output signal SD of the automatic-zero-reset amplifier 31, connected to the pressure-difference detector 22, is read.
When the device 20 is completely air tight and has no leak, the output signal SD of the amplifier 31 is ideally zero once a predetermined detection period of time has elapsed. If the device 20 has a leak, the output signal SD gradually decreases when the inside pressure is positive, and the output signal SD gradually increases when the inside pressure is negative. The output signal SD is almost proportional to the negative or positive amount of leakage in the predetermined detection period of time.
The reference value RV given by the reference-value setter 33 and the output value of the amplifier 31 are compared by the comparator 32. An acceptable/defective decision output 35 showing whether the device is acceptable or defective is obtained depending on whether the output value exceeds the reference value RV.
Even when the reference tank 21 has the same shape as the device 20 and has no leak, the pressure difference detected by the pressure-difference detector 22 is mainly influenced by the temperature difference between the device 20 and the reference tank 21 in this general leak inspector. If the device 20 and the reference tank 21 have different shapes, the pressure difference varies due to the difference in temperature between the gases in the device 20 and the reference tank 21 during a process in which the temperatures of the gases increased by the adiabatic changes caused when the gases are pressurized. Therefore, the output signal does not become zero, the ideal state. Alternatively, if the device 20 and the reference tank 21 have different temperatures, the pressure difference varies during a thermal equilibrium process after the adiabatic changes. In other words, even if the device 20 has no leak, the output signal does not become zero, the ideal state, during the predetermined detection period of time, and a pressure difference corresponding to the positive or negative amount of leakage is usually detected. This pressure difference caused by factors other than a leak is generally called a drift.
The above-described state will be explained with reference to FIG. 9. In FIG. 9, a curve A shows the drift, a curve B shows the leak, and a curve C shows the pressure difference, that is, the drift plus the leak, substantially detected by the pressure-difference detector 22. As understood from the figure, the pressure difference, indicated by the curve C, includes the drift as its major part and the leak as its minor part. As understood from the figure, the increase in the pressure difference caused by the drift approaches almost zero as time elapses. In contrast, the pressure difference caused by the leak increases almost at a constant rate as time elapses.
Focusing on this point, in the leak inspector having the structure shown in FIG. 8, the output of the automatic-zero-reset amplifier 31 is forcedly reset to zero at a certain time, TIM1 (time after the rate of increase in the drift approaches zero, shown in FIG. 9); the gain of the amplifier 31 is increased after the reset to amplify the detection signal of the pressure-difference detector 22, and the output signal SD (curve D) is sent to the comparator 32; the output signal SD obtained after a predetermined period of time is compared with the reference value RV in the comparator 32; and it is determined that the device is defective if the output signal SD exceeds the reference value RV.
With this detection method, since inspection is started after the rate of increase in the drift approaches zero, the influence of the drift can be removed. However, the inspection time for one device under inspection is as long as several tens of seconds.
To eliminate this drawback, a leak inspection method shown in FIG. 10 has been proposed. In this method, in a calibration mode, the pressure difference detected by the pressure-difference detector 22 after the pressure applying and equilibrium periods is reset to zero at regular intervals of unit detection periods, for example, by the automatic-zero-reset amplifier 31, described with reference to FIG. 8; this reset operation is repeated until a change in the pressure difference converges to within a constant range in the unit detection period; and the change Db in the pressure difference is obtained when it converges. The change Db in the pressure difference is a pressure difference change caused by an actual leak in the unit detection period.
Therefore, the drift caused in the thermal equilibrium process during the inspection period after the adiabatic change can be obtained by subtracting Db from a change Da in the pressure difference in the first unit detection period, that is, Da−Db=Dc. This value Dc is stored as a drift correction value. In an inspection mode, a pressurized gas is applied to the device 20, and the drift correction value Dc is subtracted from a change Da in the pressure difference in the first unit detection period immediately after the pressurization and equilibrium periods to obtain a change Db in the pressure difference in the unit detection period, corresponding to the actual leak of the device 20.
With the calibration method shown in FIG. 10, correct leak inspection is executed only for the temperature environment (air temperature and the temperature of the device 20) where the calibration is executed. However, in the leak inspection mode, if the room temperature or the temperature of the device 20 differs by a predetermined value or more from the temperature in the calibration mode where the drift correction value Dc is obtained, it is necessary to execute the calibration again to obtain an appropriate drift correction value Dc.
In the above description, the pressure-difference leak inspector shown in FIG. 8 has been taken as an example. A drift also occurs in another type of leak inspector (hereinafter called a gauge-pressure type leak inspector), shown in FIG. 11, in which a pressurized gas is directly applied to a device 20 under inspection; the gas pressure is measured by a pressure measuring unit 23; whether a leak exists is determined based on whether the pressure of the gas sealed in the device 20 changes by a predetermined value or more. Therefore, the gauge-pressure type leak inspector also has the same drawback as the pressure-difference leak inspector.
To eliminate the drawbacks of the pressure-difference leak inspector and the gauge-pressure type leak inspector, the applicant proposed, in Patent Literature 1, a drift-correction-coefficient calculation method, a drift correction method for correcting a drift by using a drift correction coefficient calculated by the drift-correction-coefficient calculation method, a drift-correction-coefficient learning method, and a leak inspection method and a leak inspector that use each of the methods.
In the leak inspector proposed before, in a calibration mode, a positive or negative gas pressure is applied to a device under inspection and a reference tank; changes ΔP1 and ΔP2 in pressure difference are measured when duration T1 elapses from the end of the pressure applying and equilibrium periods (see FIG. 10) and when duration T1 further elapses, respectively; a change ΔC in pressure difference, corresponding to the leak of the device, obtained when duration T1 elapses from when the temperatures of the device and the reference tank become stable is measured; and a drift correction coefficient K is calculated from the changes ΔP1, ΔP2, and ΔC using the expression K=(ΔP2−ΔC)/(ΔP1−ΔP2). In an inspection mode, changes ΔP1′ and ΔP2′ in pressure difference are measured when duration T1 elapses from the end of the pressure applying and equilibrium periods and when duration T1 further elapses, respectively; the drift correction coefficient K is used to estimate a drift J from the expression J=(ΔP1′−ΔP2′)K; and a change S in pressure difference in which the drift has been corrected, corresponding to the leak, is calculated from the expression S=ΔP2′−J.
An operation sequence in the inspection mode in the general leak inspector will be described with reference to FIG. 12. In FIG. 12, a period A1 indicates a pressure applying period, a period A2 indicates an equilibrium period, a period A3 indicates an inspection period, and a period A4 indicates a discharge period. In the pressure applying period A1, the three-way solenoid valve 14, shown in FIG. 8, is opened between a and b, and the valves 16 and 17 are also opened to apply a constant-pressure gas to the device 20 and the reference tank 21. In the equilibrium period A2, the valves 16 and 17 are closed to seal the device 20 and the reference tank 21 having the gas applied thereto, and stable gas pressure is awaited. In other words, this period is a thermal equilibrium process where the temperature of the inside gas increased by the adiabatic change in the pressure applying period A1 is gradually reduced to the temperature of the device. In the inspection period A3, it is determined whether the gas pressure that became stable in the equilibrium period A2 shows a difference. In the discharge period A4, the valves 16 and 17 are opened, and the three-way solenoid valve 14 is opened between b and c to discharge the gases sealed in the device 20 and the reference tank 21 to the atmosphere.
A curve P shown in FIG. 12 indicates a change in pressure in the device or the reference tank. The pressure abruptly increases in the pressure applying period A1, and the inside gas temperature also rises due to the adiabatic change. In the equilibrium period A2 and the inspection period A3, the inside gas temperature lowers to the temperature of the device, and the applied air pressure gradually becomes stable.
[Patent Literature 1] Japanese Patent Application Laid Open No. 2001-50854